Electrochemical treatment of heavy oil streams followed by caustic extraction or thermal treatment

ABSTRACT

This invention relates to the electrochemical conversion of dibenzothiophene type molecules of petroleum feedstreams to mercaptans that can then be removed, in one embodiment, by caustic extraction. In another embodiment, the mercaptans can be thermally decomposed, removing sulfur as hydrogen sulfide. The conversion of dibenzothiophenes to mercaptans is performed by electrochemical means without the required addition of hydrogen and in the substantial absence of water.

This Application claims the benefit of U.S. Provisional Application No.61/008,413 filed Dec. 20, 2007.

FIELD OF THE INVENTION

This invention relates to the electrochemical conversion ofdibenzothiophene type molecules of petroleum feedstreams to mercaptansthat can then be removed, in one embodiment, by caustic extraction. Inanother embodiment, the mercaptans can be thermally decomposed, removingsulfur as hydrogen sulfide. The conversion of dibenzothiophenes tomercaptans is performed by electrochemical means without the requiredaddition of hydrogen and in the substantial absence of water.

BACKGROUND OF THE INVENTION

The sulfur content of petroleum products is continuing to be regulatedto lower and lower levels throughout the world. Sulfur specifications inmotor gasoline (“mogas”) and on-road diesel have been most recentlyreduced and future specifications will further lower the allowablesulfur content of off-road diesel and heating oils. Sulfur is currentlyremoved from petroleum feedstreams by various processes depending on thenature of the feedstream. Processes such as coking, distillation, andalkali metal dispersions are primarily used to remove sulfur from heavyfeedstreams, such as bitumens which are complex mixtures and typicallycontain hydrocarbons, heteroatoms, and metals, with carbon chains inexcess of about 2,000 carbon atoms. For lighter petroleum feedstreams,such as distillates, catalytic hydrodesulfurization is typically used.The sulfur species in such feedstreams span a range of molecular typesincluding sulfides, thiols, thiophenes, benzothiophenes todibenzothiophenes in order of decreasing hydrodesulfurization (HDS)reactivity. The most difficult to remove sulfur is found in stericallyhindered dibenzothiophene molecules such as diethyl dibenzothiophene.The space velocity, temperature and hydrogen pressures of catalytic HDSunits are determined primarily by the slow reaction kinetics of theserelatively minor components of the feed. These are the molecules thatare typically left in the product after conventional low-pressurehydrotreating. Further removing these molecules often requires higherhydrogen pressure and higher temperature (“deep desulfurization”) whichleads to higher hydrogen consumption and shorter catalyst run lengthswhich are costly results. Therefore, it is desirable to have alternativeprocesses that are capable of removing these refractory sulfur moleculeswithout incurring more severe reaction conditions for catalytichydrotreating, which can result in significant capital and energysavings.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention thereis provided a process for removing sulfur from petroleum feedstreamscontaining sulfur in the form of hindered dibenzothiophene compounds,comprising:

a) passing a sulfur-containing petroleum feedstream to anelectrochemical cell;

b) subjecting said feedstream to an effective voltage and current thatwill result in the conversion of at least a portion of said hindereddibenzothiophene compounds to mercaptan compounds;

c) passing the electrochemically treated petroleum feedstream containingsaid mercaptans compounds to a caustic treatment zone wherein it iscontacted with an aqueous caustic solution wherein mercaptan-containingcompounds are extracted by the aqueous caustic solution; and

d) collecting a reduced-sulfur petroleum product stream from the caustictreatment zone;

wherein the reduced-sulfur petroleum product stream has a lower sulfurcontent by wt % than the sulfur-containing petroleum feedstream.

In a preferred embodiment, the sulfur-containing petroleum feedstream iscomprised of a bitumen.

In another preferred embodiment, the feedstream is a distillate boilingrange hydrocarbon stream and an effective amount of an electrolyte ismixed with the distillate boiling range stream to be treated.

Also in accordance with another preferred embodiment of the presentinvention is a process for removing sulfur from petroleum feedstreamscontaining sulfur in the form of hindered dibenzothiophene compounds,comprising:

a) passing a sulfur-containing petroleum feedstream to anelectrochemical cell;

b) subjecting said feedstream to an effective voltage and current thatwill result in the conversion of at least a portion of said hindereddibenzothiophene compounds to mercaptan compounds;

c) passing the electrochemically treated petroleum feedstream containingmercaptan compounds to a thermal decomposition zone wherein at least aportion of the mercaptans are decomposed to hydrogen sulfide attemperatures from about 302° F. to about 932° F. (150° C. to 500° C.);and

d) collecting a reduced-sulfur petroleum product stream from the thermaldecomposition zone;

wherein the reduced-sulfur petroleum product stream has a lower sulfurcontent by wt % than the sulfur-containing petroleum feedstream.

In a preferred embodiment, the sulfur-containing petroleum feedstream iscomprised of a bitumen.

In another preferred embodiment, the feedstream is a distillate boilingrange stream and an effective amount of an electrolyte is mixed with thedistillate boiling range stream to be treated.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 hereof is a plot of conductivity versus temperature for variouspetroleum residues and crudes.

DETAILED DESCRIPTION OF THE INVENTION

Feedstreams suitable for use in the present invention range from heavyoil feedstreams, such as bitumens to those boiling in the distillaterange. In a preferred embodiment the heavy oil feedstream contains atleast about 10 wt. %, preferably at least about 25 wt. % of materialboiling above about 1050° F. (565° C.), both at atmospheric pressure (0psig). Such streams include bitumens, heavy oils, whole or topped crudeoils and residua. The bitumen can be whole, topped or froth-treatedbitumen. Non-limiting examples of distillate boiling range streams thatare suitable for use herein include diesel fuels, jet fuels, heatingoils, kerosenes, and lubes. Such streams typically have a boiling rangefrom about 302° F. (150° C.) to about 1112° F. (600° C.), preferablyfrom about 662° F. (350° C.) to about 1022° F. (550° C.). Otherpreferred streams are those typically known as Low Sulfur AutomotiveDiesel Oil (“LSADO”). LSADO will typically have a boiling range of about350° F. (176° C.) to about 550° F. (287° C.) and contain from about 200wppm sulfur to about 2 wppm sulfur, preferably from about 100 wppmsulfur to about 10 wppm sulfur. The process embodiments of the presentinvention electrochemically treat a sulfur-containing petroleumfeedstream resulting in a reduced-sulfur petroleum product stream whichhas a lower sulfur concentration by wt % than the sulfur-containingpetroleum feedstream.

A major of the sulfur contained in heavy oils and distillates are in theform of hindered dibenzothiophene molecules. Although such molecules aredifficult to remove by conventional hydrodesulfurization processeswithout using severe conditions, such as high temperatures andpressures, such molecules are converted by the practice of the presentinvention to sulfur species that are more easily removed by conventionalnon-catalytic processes. For example, the electrochemical step of thepresent invention converts the hindered dibenzothiophene (“DBT”)molecules, which are substantially refractory to conventionalhydrodesulfurization, to hydrogenated naphthenobenzothiophene mercaptanmolecules that are more readily extracted with use of caustic solutionor by thermal decomposition. This capability can significantlydebottleneck existing distillate hydrotreating process units byconverting the slowest to convert molecules (hindered dibenzothiophenes)into much more readily extractable mercaptan species, preferablyalkylated biphenyl mercaptan species.

The process of the present invention does not require the addition of anelectrolyte when heavy oil is the feedstream, but rather, relies on theintrinsic conductivity of the heavy oil at elevated temperatures. Itwill be understood that the term “heavy oil” and “heavy oil feedstream”as used herein includes both bitumen and other heavy oil feedstreams,such as crude oils, atmospheric resids, and vacuum resids. This processis preferably utilized to upgrade bitumens and/or crude oils that havean API gravity of less than about 15. The inventors hereof haveundertaken studies to determine the electrochemical conductivity ofcrudes and residues at temperatures up to about 572° F. (300° C.) andhave demonstrated an exponential increase in electrical conductivitywith temperature as illustrated in FIG. 1 hereof. It is believed thatthe electrical conductivity in crudes and residues is primarily carriedby electron-hopping in the π-orbitals of aromatic and heterocyclicmolecules. Experimental support for this is illustrated by the simpleequation, shown in FIG. 1 hereof, that can be used to calculate theconductivity of various cuts of a crude using only its temperaturedependent viscosity and its Conradson carbon content. The molecules thatcontribute to Concarbon are primarily the large multi-ring aromatic andheterocyclic components.

A 4 mA/cm² electrical current density at 662° F. (350° C.) with anapplied voltage of 150 volts and a cathode-to-anode gap of 1 mm wasmeasured for an American crude oil. Though this is lower than would beutilized in preferred commercial embodiments of the present invention,the linear velocity for this measurement was lower than the preferredvelocity ranges by about three orders of magnitude: 0.1 cm/s vs. 100cm/s. Using a 0.8 exponent for the impact of increased flow velocity oncurrent density at an electrode, it is estimated that the currentdensity would increase to about 159 mA/cm² at a linear velocity of about100 cm/s. This suggests that more commercially attractive currentdensities achieved at higher applied voltages. Narrower gap electrodedesigns or fluidized bed electrode systems could also be used to lowerthe required applied voltage.

Unlike bitumen, performing controlled potential electrolysis on anon-conductive fluid such as a LSADO, or other petroleum distillatestreams, requires the introduction of an effective amount of anelectrolyte, such as a conductive salt. There is an insufficientconcentration of large multi-ring aromatic and heterocyclic molecules indistillate boiling range feedstreams to produce sufficient intrinsicconductivity without the use of an electrolyte. The direct addition of aconductive salt to the distillate feedstream can be difficult forseveral reasons. The term “effective amount of electrolyte” as usedherein means at least than amount needed to produce conductivity betweenthe anode and the cathode of the electrochemical cell. Typically thisamount will be from about 0.5 wt. % to about 50 wt. %, preferably fromabout 0.5 wt. % to about 10 wt. %, of added electrolytic material basedon the total weight of the feed plus the electrolyte. Once dissolved inthe oil, most salts are difficult to remove after electrolysis.Incomplete salt removal is unacceptable due to product specifications,negative impact on further catalytic processing, potential corrosivityand equipment fouling. Even salts that are soluble in a low dielectricmedium are often poorly ionized and therefore unacceptable highconcentrations are required to achieve suitable conductivities. Inaddition, such salts are typically very expensive. However, recentadvances in the field of ionic liquids have resulted in new organicsoluble salts having melting points lower than about 212° F. (100° C.)that can be used in the present invention. They can be recovered bysolvent washing the petroleum stream after electrolysis. Non-limitingexamples of such salts include: 1-butyl-1-methylpyrrolidiniumtris(pentafluoroethyl)trifluoro phosphate, 1-butyl-1-methylpyrrolidinium trifluoro-methyl sulfonated, trihexyltetradecylphosphoniumtris(pentafluoroethyl)trifluorophosphate andethyl-dimethylpropyl-ammonium bis(trifluoro-methylsulfonyl)imide.

An alternate solution to the low conductivity problem of distillateboiling range feedstreams to produce a two phase system. Rather thanadding an electrolyte to the feedstream, the feedstream can be dispersedin a conductive, immiscible, non-aqueous electrolyte. Such a two-phasesystem of oil dispersed in a continuous conductive phase provides asuitable electrolysis medium. The continuous conductive phase providesthe sufficient conductivity between the cathode and anode of anelectrochemical cell to maintain a constant electrode potential.Turbulent flow through the electrochemical cell brings droplets of thefeedstream in contact with the cathode, at which point electrons aretransferred from the electrode to sulfur containing species on thedroplet surface.

After reaction, the immiscible electrolyte from the treated feedstreamis separated by any suitable conventional means resulting in a reducedsulfur product stream. The immiscible electrolyte can be recycled. Theelectrolyte in the immiscible electrolysis medium is preferably anelectrolyte that dissolves, or dissociates, in the solvent to produceelectrically conducting ions, but that does not undergo a redox reactionin the range of the applied potentials used. Suitable organicelectrolytes for use in the present invention, other than thosepreviously mentioned, include quaternary carbyl- and hydrocarbyl-oniumsalts, e.g., alkylammonium hydroxides. Non-limiting examples ofinorganic electrolytes include, e.g., NaOH, KOH and sodium phosphates,and mixtures thereof. Non-limiting examples of onium ions that can beused in the practice of the present invention include mono- andbis-phosphonium, sulfonium and ammonium, preferably ammonium. Preferredcarbyl and hydrocarbyl moieties are alkyl carbyl and hydrocarbylmoieties. Suitable quaternary alkyl ammonium ions include tetrabuytylammonium, and tetrabutyl ammonium toluene sulfonate. Optionally,additives known in the art to enhance performance of the electrodes canalso be used. Non-limiting examples of such additives suitable for useherein include surfactants, detergents, emulsifying agents and anodicdepolarizing agents. Basic electrolytes are most preferred. Theconcentration of salt in the electrolysis medium should be sufficient togenerate an electrically conducting solution in the presence of thefeedstream. Typically, a concentration of about 1 to about 50 wt %conductive phase, preferably about 5 to about 25 wt % based on theoverall weight of the oil/water/electrolyte mixture is suitable. It ispreferred that petroleum stream immiscible solvents be chosen, such asdimethyl sulfoxide, dimethylformamide or acetonitrile.

Dispersions are preferred for ease of separation following electrolysis.However, more stable oil-in-solvent emulsions can also be used.Following electrolytic treatment, the resulting substantially stableemulsion can be broken by the addition of heat and/or a de-emulsifyingagent.

The electrochemical cell used in the practice of the present inventionmay be divided or undivided. Such systems include stirred batch or flowthrough reactors. The foregoing may be purchased commercially or madeusing technology known in the art. Suitable electrodes known in the artmay be used. Included as suitable electrodes are three-dimensionalelectrodes, such as carbon or metallic foams. The optimal electrodedesign would depend upon normal electrochemical engineeringconsiderations and could include divided and undivided plate and framecells, bipolar stacks, fluidized bed electrodes and porous threedimensional electrode designs; see Electrode Processes andElectrochemical Engineering by Fumio Hine (Plenum Press, New York 1985).While direct current is typically used, electrode performance may beenhanced using alternating current or other voltage/current waveforms.The gap between electrode surfaces will preferably be about 1 to about50 mm, more preferably from about 1 to about 25 mm, and the linearvelocity in the electrochemical cell will be in the range of about 1 toabout 500 cm/s, more preferably in the range of about 50 to about 200cm/s.

The applied cell voltage, that is, the total voltage difference betweenthe cathode and anode will vary depending upon the cell design andelectrolytes used. What is critical, however, is that the cathode bepolarized sufficiently to achieve electron transfer to thedibenzothiophene molecules, which occurs at reduction potentials morenegative than −2.3 Volts versus a standard calomel electrode. Normalelectrochemical practices can be used to ensure that the cell isoperated under these conditions. In preferred embodiments, the voltageacross the electrochemical cell will be about 4 to about 500 volts,preferably from about 100 to about 200 volts, with a resulting currentdensity of about 10 mA/cm² to about 1000 mA/cm², preferably from about100 mA/cm² to about 500 mA/cm².

At least a portion of the hindered dibenzothiophene compounds in thefeedstream are converted to the corresponding alkylated biphenylmercaptan compounds in the electrochemical cell. The mercaptanscontaining treated feedstream is passed to a caustic wash step whereinit is contacted with an aqueous caustic solution for extraction of themercaptan species. Any suitable caustic wash technology can be used inthe practice of the present invention. The most preferred caustic washwould be an aqueous solution of sodium hydroxide having a strength fromabout 0.5 M to about 5 M and mixing the mercaptan-containing stream withair and the caustic solution to remove the mercaptan species in thecaustic solution. Non-limiting examples of caustic extraction processesthat can be used in the practice of the present invention include theUOP® MEROX® process and the Merichem® THIOLEX® and EXOMER® processes.The MEROX® Process was announced to the industry in 1959. The Oil & GasJ. 57(44), 73-8 (1959) contains a discussion of the MEROX® Process. Inthe MEROX® oxidation process, mercaptan compounds are extracted from thefeed and then oxidized by air in the caustic phase in the presence ofthe MEROX® catalyst, which is typically an iron group chelate (cobaltphthalocyanine) to form disulfides which are then redissolved in thehydrocarbon phase, leaving the process as disulfides in the hydrocarbonproduct. The disulfides, which are not soluble in the caustic solution,can be separated and recycled for mercaptan extraction. The treatedstream is usually sent to a water wash in order to reduce the sodiumcontent.

All of these processes take advantage of the acidity of the mercaptanspecies. By contacting a petroleum stream that contains acidic mercaptanspecies with an aqueous base solution, the mercaptans are de-protonated,converted to salts and are now more soluble in the aqueous stream andthus can be extracted nearly quantitatively from the petroleum stream.Such an extraction is ineffective with the original, non-acidicdibenzothiophenic sulfur species. The desulfurized petroleum stream isthen separated from the resulting mercaptide containing causticsolution. The caustic solution can then be regenerated and themercaptides isolated in a variety of conventional ways depending on theprocess design. Such mercaptan extractions are widely used in thepetroleum refining industry and it is likely that every refinery has atleast one such unit. The extracted mercaptans can be readily oxidized todisulfides, separated from the caustic stream, and recycled for moremercaptan extraction. The hindered dibenzothiophene (“DBT”) specieswhich are removed from the feedstream are converted to a relativelysmall substantially pure stream of disulfides that can be disposed ofvia combustion. They can also be fed to a coking unit for thermaldecomposition. Being able to target hindered DBT molecules can alsoenable the disposition of Light Catalytic Cycle Oil (“LCCO”), which isrich in DBTs, to distillate hydrotreaters.

In a second embodiment of the present invention, following, orsimultaneous with the electrochemical conversion of thedibenzothiophenic species to mercaptans, a thermal decompositionreaction of the mercaptans is performed to decompose them with loss ofhydrogen sulfide from the mercaptan molecule. This thermal decompositioncan be performed at temperatures from about 302° F. to about 932° F.(150° C. to 500° C.), preferably from about 482° F. to about 932° F.(250° C. to 500° C.) and at ambient to autogenous pressure. Subsequentremoval of this hydrogen sulfide from the petroleum stream will producea reduced sulfur product stream that is lower is sulfur content by wt %than the sulfur-containing petroleum feedstream treated by the currentprocess.

The present invention will be better understood with reference to thefollowing examples which are presented for illustrative purposes and arenot to be taken as limiting the invention in anyway.

The following three examples were performed using a 300-cc autoclave(Parr Instruments, Moline, Ill.) was modified to allow two insulatingglands (Conax, Buffalo, N.Y.) to feed through the autoclave head. Twocylindrical stainless steel (316) mesh electrodes are connected to theConax glands, where the power supply (GW Laboratory DC Power Supply,Model GPR-1810HD) is connected to the other end. The autoclave body isfitted with a glass insert, a thermal-couple and a stirring rod. Theautoclave can be charged with desired gas under pressure and run eitherin a batch- or a flow-through mode.

EXAMPLE 1 Electrochemical Treatment of DBT Under N₂ in DimethylSulfoxide Solvent with Tetrabutylammonium HexaflouorphosphateElectrolyte

To the glass insert was added 1.0 g dibenzothiophene (“DBT”), 3.87 gtetrabutylammonium hexafluorophosphate (TBAPF₆), and 100 milliliter(“ml”) anhydrous dimethyl sulfoxide (DMSO, Aldrich). After the contentwas dissolved, the glass insert was loaded into the autoclave body, theautoclave head assembled and pressure tested. The autoclave was chargedwith 70 psig of N₂ and heated to 212° F. (100° C.) with stirring (300rpm). A voltage of 5 Volts was applied and the current was 0.8 Amp. Thecurrent gradually decreased with time and after two hours, the run wasstopped. The autoclave was opened and the content acidified with 10% HCl(50 ml). The acidified solution was then diluted with 100 ml ofde-ionized (“DI”) water, extracted with ether (50 ml×3). The ether layerwas separated and dried over anhydrous Na₂SO₄, and ether was allowed toevaporate under a stream of N₂. The isolated dry products were analyzedby GC-MS. A conversion of 12% was found for DBT and the products are asthe following.

This example shows that the electrochemical reduction of DBT under N₂resulted in: 12% DBT conversion after 2 h at 212° F. GC-MS revealed thatthe products consisted of 35% 2-phenyl benzenethiol, 8% tetrahydro-DBT,and 57% of a species with a mass of 214. The assignment of this peak as2-phenyl benzenethiol was done by comparing with an authentic sample.The mass 214 species was tentatively assigned as 2-phenyl benzenethiolwith two methyl groups added. Addition of methyl groups to DBT indicatesthat decomposition of solvent DMSO occurred since it is the only sourceof methyl groups in this system. No desulfurization product biphenyl wasobserved in this run.

COMPARATIVE EXAMPLE A Electrochemical Treatment of DBT Under Hydrogen inDimethyl Sulfoxide Solvent with Tetrabutylammonium HexaflouorphosphateElectrolyte

To the glass insert was added 0.5 g dibenzothiophene (“DBT”), 3.87 gtetrabutylammonium hexafluorophosphate (TBAPF₆), and 100 ml anhydrousdimethyl sulfoxide (DMSO, Aldrich). After the content was dissolved, theglass insert was loaded into the autoclave body, the autoclave headassembled and pressure tested. The autoclave was charged with 300 psigof H₂ and heated to 257° F. (125° C.) with stirring (300 rpm). A voltageof 4.5 Volts was applied and the current was 1.0 Amp. The currentgradually decreased with time and after three and half (3.5) hours, therun was stopped. The autoclave was opened and the content acidified with10% HCI (50 ml). The acidified solution was then diluted with 100 ml ofDI water, extracted with ether (50 ml×3). The ether layer was separatedand dried over anhydrous Na₂SO₄, and ether was allowed to evaporateunder a stream of N₂. The isolated dry products were analyzed by GC-MS.A conversion of 16.5% was found for DBT and the products are as thefollowing.

COMPARATIVE EXAMPLE B Electrochemical Treatment of DEDBT Under Hydrogenin Dimethyl Sulfoxide Solvent with TetrabutylammoniumHexaflouorphosphate Electrolyte

To the glass insert was added 1.0 g 4,6-diethyl dibenzothiophene(“DEDBT”), 3.87 g tetrabutylammonium hexafluorophosphate (TBAPF₆), and100 ml anhydrous dimethyl sulfoxide (DMSO, Aldrich). After the contentis dissolved, the glass insert was loaded into the autoclave body, theautoclave head assembled and pressure tested. The autoclave was chargedwith 200 psig of H₂ and heated to 100° C. with stirring (300 rpm). Avoltage of 7 Volts was applied and the current was 1.0 Amp. The currentgradually decreased with time and after two and half (2.5) hours, therun was stopped. The autoclave was opened and the content acidified with10% HCl (50 ml). The acidified solution was then diluted with 100 ml ofDI water, extracted with ether (50 ml×3). The ether layer was separatedand dried over anhydrous Na₂SO₄, and ether was allowed to evaporateunder a stream of N₂. The isolated dry products were analyzed by GC-MS.A conversion of 16% was found for DEDBT and the products are as thefollowing.

Similarly, desulfurization was also observed for sterically hinderedDiethyl Dibenzothiophene (DEDBT) under H₂. A conversion of 16% of theDEDBT was observed and the products contained 53% desulfurizedcompounds, 46% dihydro-DEDBT and a trace amount of tetrahydro-DEDBT.Solvent decomposition also occurs in this case. Although electrochemicaldesulfurization of DBT and hindered DBT has been achieved under H₂ inthe 212° F. to 257° F. (100° C. to 125° C.) temperature range, theconversion is still quite low. Increased conversions were attempted byextending the run time by operating within this temperature range or byrunning at higher temperature of about 392° F. (200° C.) to about 482°F. (250° C.).

The first example illustrates that DBT's can be readily converted intoalkylated biphenyl mercaptans electrochemically without the addition ofhydrogen or water. The mercaptans can be removed by caustic extraction.For example, standard MEROX® caustic treatment could be used to removethese molecules from the electro-treated LSADO producing ultra-lowsulfur distillate without the need for additional hydrotreatment. Due tothe low concentration of these molecules in the LSADO, the powerconsumption should be minimal. The comparative examples demonstratethat, electrochemical reduction in the presence of hydrogen leads toproduction of hydrogenated naphtheno dibenzothiophenes and not biphenylmercaptans. These species are not caustic extractable. By limiting theavailability of hydrogen sources by eliminating the hydrogen or watercontent, the products of the electrolysis can be controlled. Thechemistry of conversion to biphenyl mercaptans and subsequent extractionprocesses are as follows:

EXAMPLE 2 Thermal Decomposition of 2-Phenylthiophenol in Tetralin at400° C.

A volume of 1.5 ml of a tetralin solution containing 0.1 M of2-phenylthiopheol was placed into 3 ml stainless-steel mini-bomb insidea dry-box. The mini-bomb was heated at 400° C. in an oven for a certainperiod of time and the content analyzed by GC/MS. Results in Table 1below indicate desulfurization of 2-phenylthiophenol, giving biphenyl asthe major product.

EXAMPLE 3 Thermal Decomposition of 2-Phenylthiophenol in Tetralin at375° C.

A volume of 1.5 ml of a tetralin solution containing 0.1 M of2-phenylthiopheol was placed into 3 ml stainless-steel mini-bomb insidea dry-box. The mini-bomb was heated at 375° C. in an oven for a certainperiod of time and the content analyzed by GC/MS. Results in Table 1below indicate desulfurization of 2-phenylthiophenol, giving biphenyl asthe major product.

EXAMPLE 4 Thermal Decomposition of 2-Phenylthiophenol in Tetralin at350° C.

A volume of 1.5 ml of a tetralin solution containing 0.1 M of2-phenylthiopheol was placed into 3 ml stainless-steel mini-bomb insidea dry-box. The mini-bomb was heated at 350° C. in an oven for a certainperiod of time and the content analyzed by GC/MS. Results in Table 1below indicate desulfurization of 2-phenylthiophenol, giving biphenyl asthe major product. Based on the thermal decomposition rates at varioustemperatures, the activation energy for 2-phenylthiophenol thermaldecomposition was determined to be ˜29.2 kcal/mol.

EXAMPLE 5 Thermal Decomposition of Phenyl Disulfide in Tetralin at 300°C.

A volume of 1.5 ml of a tetralin solution containing 0.1 M of phenyldisulfide (PhS—SPh) was placed into 3 ml stainless-steel mini-bombinside a dry-box. The mini-bomb was heated at 572° F. (300° C.) in anoven for 4h and the content analyzed by GC/MS. All disulfide isconverted into thiophenol. By analogy, biphenyl disulfide(Ph-Ph-S—S-Ph-Ph) can be converted into 2-phenylthiophenol, which can bedesulfurized at higher temperature as shown in Examples 2 through 4herein. Equation 5 illustrates the thermal conversion of2-phenylthiophenol to biphenyl and hydrogen sulfide.

TABLE 1 Thermal Decomposition of 2-Phenylthiophenol (0.1 M) in TetralinTemp. (° C.) Time (h)

400 0  100%   0     0     0   2 22.1% 60.4%   4% 12.5% 4 29.3%   53%4.7%   12% 375 1 83.6% 11.9% 1.3%  3.1% 3 59.7%   31% 3.8%  5.4% 350 195.1%  3.6%  1.3% 4 72.6% 17.4% 5.7%  4.3%

As Examples 2 through 5 clearly demonstrate, the biphenyl mercaptan canbe desulfurized by thermal treatment. This reaction could occursimultaneously with electrochemical processing if conducted atsufficiently elevated temperatures or may require a separate thermalsoak step.

1. A process for removing sulfur from a sulfur-containing petroleumfeedstream having at least a portion of its sulfur in the form ofhindered dibenzothiophene compounds, comprising: a) passing asulfur-containing petroleum feedstream to an electrochemical cell; b)subjecting said feedstream to an effective voltage and current that willresult in the conversion of at least a portion of said hindereddibenzothiophene compounds to mercaptan compounds; c) passing theelectrochemically treated petroleum feedstream containing saidmercaptans compounds to a caustic treatment zone wherein it is contactedwith an aqueous caustic solution wherein mercaptan-containing compoundsare extracted by the aqueous caustic solution; and d) collecting areduced-sulfur petroleum product stream from the caustic treatment zone;wherein the reduced-sulfur petroleum product stream has a lower sulfurcontent by wt % than the sulfur-containing petroleum feedstream.
 2. Theprocess of claim 1, wherein the sulfur-containing petroleum feedstreamis comprised of a bitumen.
 3. The process of claim 2, wherein theelectrochemical cell is run at about 4 volts to about 500 volts and acurrent density of about 10 to about 1000 mA/cm².
 4. The process ofclaim 3, wherein the aqueous caustic solution is a sodium hydroxidesolution.
 5. The process of claim 4, wherein the sulfur-containingpetroleum feedstream is comprised of a bitumen.
 6. The process of claim1, wherein the sulfur-containing petroleum feedstream is a distillateboiling range hydrocarbon stream and an effective amount of anelectrolyte is mixed with the mixture of water and distillate boilingrange hydrocarbon stream.
 7. The process of claim 6, wherein thedistillate boiling range hydrocarbon stream is a low sulfur automotivediesel oil.
 8. The process of claim 6, wherein the electrolyte is anorganic electrolyte.
 9. The process of claim 8, wherein the organicelectrolyte is selected from quaternary carbyl- and hydrocarbyl-oniumsalts.
 10. The process of claim 8, wherein the organic electrolyte iscomprised of an organic soluble salt selected from the group consistingof 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, 1-butyl-1-methyl pyrrolidinium trifluoro-methyl sulfonated,trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphateand ethyl-dimethylpropyl-ammonium bis(trifluoro-methylsulfonyl)imide.11. The process of claim 8, wherein the electrolyte is an inorganicelectrolyte selected from the group consisting of sodium hydroxide,potassium hydroxide and sodium phosphates.
 12. The process of claim 6,wherein the electrochemical cell is run at about 4 volts to about 500volts and a current density of about 10 to about 1000 mA/cm².
 13. Theprocess of claim 12, wherein the aqueous caustic solution is a sodiumhydroxide solution.
 14. The process of claim 12, at least a portion ofthe mercaptan compounds are alkylated biphenyl mercaptan compounds. 15.A process for removing sulfur from a sulfur-containing petroleumfeedstream having at least a portion of its sulfur in the form ofhindered dibenzothiophene compounds, which method comprising: a) passinga sulfur-containing petroleum feedstream to an electrochemical cell; b)subjecting said feedstream to an effective voltage and current that willresult in the conversion of at least a portion of said hindereddibenzothiophene compounds to mercaptan compounds; c) passing theelectrochemically treated petroleum feedstream containing mercaptancompounds to a thermal decomposition zone wherein at least a portion ofthe mercaptans are decomposed to hydrogen sulfide at temperatures fromabout 302° F. to about 932° F. (150° C. to 500° C.); and d) collecting areduced-sulfur petroleum product stream from the thermal decompositionzone; wherein the reduced-sulfur petroleum product stream has a lowersulfur content by wt % than the sulfur-containing petroleum feedstream.16. The process of claim 15, wherein the sulfur-containing petroleumfeedstream is comprised of a bitumen.
 17. The process of claim 16,wherein the electrochemical cell is run at about 4 volts to about 500volts and a current density of about 10 to about 1000 mA/cm².
 18. Theprocess of claim 17, wherein the thermal decomposition temperature isfrom about 482° F. to about 932° F. (250° C. to 500° C.).
 19. Theprocess of claim 18, wherein the sulfur-containing petroleum feedstreamis comprised of a bitumen.
 20. The process of claim 15, wherein thesulfur-containing petroleum feedstream is a distillate boiling rangehydrocarbon stream and an effective amount of an electrolyte is mixedwith the mixture of water and distillate boiling range hydrocarbonstream.
 21. The process of claim 20, wherein the distillate boilingrange hydrocarbon stream is a low sulfur automotive diesel oil.
 22. Theprocess of claim 20, wherein the electrolyte is an organic electrolyte.23. The process of claim 22, wherein the organic electrolyte is selectedfrom quaternary carbyl- and hydrocarbyl-onium salts.
 24. The process ofclaim 22, wherein the organic electrolyte is selected from the organicsoluble salt is selected from the group consisting of1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluoro phosphate,1-butyl-1-methyl pyrrolidinium trifluoro-methyl sulfonated,trihexyltetradecylphosphonium tris(pentafluoroethyl)trifluorophosphateand ethyl-dimethylpropyl-ammonium bis(trifluoro-methylsulfonyl)imide.25. The process of claim 20, wherein the electrolyte is an inorganicelectrolyte selected from the group consisting of sodium hydroxide,potassium hydroxide and sodium phosphates.
 26. The process of claim 20,wherein the electrolyte is an organic electrolyte selected fromquaternary carbyl- and hydrocarbyl-onium salts.
 27. The process of claim20, wherein the electrochemical cell is run at about 4 volts to about500 volts and a current density of about 10 to about 1000 mA/cm². 28.The process of claim 27, at least a portion of the mercaptan compoundsare alkylated biphenyl mercaptan compounds.
 29. The process of claim 27,wherein the thermal decomposition temperature is from about 482° F. toabout 932° F. (250° C. to 500° C.).